The present invention is directed to a method and apparatus for treating wastewater and, in particular, to a method and apparatus of measuring the chemical dosage required to reduce a gas, such as hydrogen sulfide in water.
Hydrogen sulfide gas (H2S) has an obnoxious odor often referred to as the smell of “rotten eggs”. It is toxic at relatively low concentrations and also extremely corrosive as it forms an acidic solution when dissolved in water. These characteristics make hydrogen sulfide a major problem for many wastewater collection and treatment facilities where costly chemicals must be dosed in order to combat its effects.
Hydrogen sulfide gas is a product of natural biological processes that occur when the dissolved oxygen levels in wastewater containing organic matter is depleted by oxygen consuming bacteria. Certain species of bacteria have the ability to meet their oxygen requirement by stripping oxygen from inorganic ions present in the water. When the stream contains sulfate (SO42−) or sulfite (SO32−) ions, this results in the release of sulfide (S2−) ions into the stream.(SO42−+SO32−)+bacteria+organic material----->S2−+more bacteria
Equilibrium is established between dissolved hydrogen sulfide gas (H2S), bisulfide ions (HS−), and sulfide ions (S2−) that are dependent upon pH.H2S<==>H++HS−<==>H++S2−
These three forms of sulfides make up the “Dissolved Sulfides” in the waste stream. The dissolved hydrogen sulfide gas escapes through the liquid/air interface into the air. The rate per unit area of exposed surface at which it escapes depends upon temperature and the concentration gradient across the air/water interface; i.e., the difference between the concentrations of dissolved H2S gas in the water and the concentration in the air. The concentration of dissolved hydrogen sulfide gas in wastewater determines the concentration of hydrogen sulfide gas immediately above the water surface. How this is detected as odor depends upon the air movement above the water surface, and the actual surface area. The latter depends not only on geometry but is also influenced by agitation caused by wind and mechanical actions.
Various approaches have been developed in order to reduce or eliminate hydrogen sulfide gas from collections systems, pump stations, and wastewater treatment plants. A common approach is to dose chemicals, which either eliminate the formation of sulfides, eliminate dissolved sulfides once they have been produced, or do both. Often iron salts (ferric chloride, ferrous chloride, ferrous sulfate) are added to the waste stream resulting in the precipitation of insoluble ferrous sulfide. Such bound sulfides play no further role in odor release. Bioxide, a proprietary form of calcium nitrate, inhibits the formation of sulfides by offering the bacteria an alternative source of oxygen. They prefer stripping oxygen from nitrate ions over stripping oxygen from sulfate or sulfide ions. Bioxide also has been shown to reduce dissolved sulfides already present in the stream.
The most common approach by wastewater treatment plant operators is to take grab samples and measure total or dissolved sulfides, such as by using a test kit of the type marketed by LaMotte or Hach. Using the result obtained, they adjust the rate of chemical feed based upon historical experience. Grab samples may be taken once per day or at less frequent intervals. Operators develop a sense as to when higher dosage levels may be required. A common operating procedure is to add chemicals by two fixed rate pumps. One will be run continuously while the second is turned on and off, either manually or by using a timer, to coincide with times perceived to require higher dosing rates.
This approach often results in overdosing—costly because of higher usage than necessary—and under-dosing—costly because of odor complaints and infrastructure damage. The conditions that influence sulfide formation change from hour-to-hour as well as day-to-day; hence, real-time monitoring is highly desirable.
Another approach is a total sulfides analyzer which acidifies the sample in order to convert all dissolved sulfides present into dissolved H2S gas. This is then sparged from the system using air and the concentration of H2S gas in the airstream is measured by a gas phase sensor. The process of acidification releases sulfide ions from “bound” sulfides—sulfides that exist within the stream in particulate form—not in ionic form. These do not contribute to odor and should not be considered when assessing the potential of a waste stream to produce odor. Total sulfides is not the best parameter for the efficient dosing of odor chemicals as, in many instances, it will overestimate the potential of the stream to release H2S.
In another approach, the sample is not acidified. The monitor calculates a new parameter—“purgeable sulfides”. The relationship between this parameter and dissolved sulfides has not been established.